Electrode materials which are transparent to light have been widely used for electrodes in display devices, such as liquid crystal display devices, electroluminescense, etc., and photosensitive devices, such as photo-cells, image pick-up tubes, etc., as well as anti-clouding or anti-freezing electrodes for window glass of automobiles, aeroplanes, buildings and the like.
As such transparent electrode materials, a tin oxide-antimony oxide type, an indium oxide-tin oxide type and the like are well known. These metal oxides can form a film on a glass or ceramic plate to provide a transparent conductive film.
The well-known methods for forming a transparent conductive film includes (1) a vacuum evaporation method, (2) a sputtering method, (3) a CVD method, and (4) a coating method.
However, each of the above-described methods (1), (2) and (3) has problems in that: equipments used therefor are so complicated that these methods are inferior in processability; and in the usual case, etching must be carried out after film formation in order to form patterns.
The above-described method (4) offers a possibility of solving the aforesaid problems associated with the methods (1) to (3), but there still is a problem that it is difficult to obtain a transparent conductive thin film which can withstand practical use.
Methods which have been proposed for effecting the above method (4) include a method of using solutions of inorganic salts, e.g., InCl.sub.3, In(NO.sub.3).sub.3, SnCl.sub.4, etc., in organic solvents, a method of using indium salts of organic acids having a strong ionic bond, such as indium octylate, etc., a method of using organic complexes of indium or tin, and the like. These methods, however, involve disadvantages such as hindering of homogenization of the film or gellation of the coating composition upon thermal decomposition of the coated film formed on the substrate plate, thus causing non-uniformity or turbidity of the film or insufficient mechanical strength to easily get scratches.
It is hitherto known that an indium oxide composition is produced by liquid phase neutralization of an inorganic indium salt with an alkaline substance, such as sodium hydroxide or aqueous ammonia, as disclosed in West Germany Patent Publication (OLS) No. 2,458,508. However, the indium oxide composition produced by the above-described known method contains salts as by-products, e.g., NaCl, NaNO.sub.3, NH.sub.4 Cl, NH.sub.4 NO.sub.3, etc., and such salts prevent the film from homogenization when the composition is coated on a substrate plate such as glass and then heat-baked or the by-produced salts remain in the film after baking.